Integrating active matrix inorganic light emitting diodes for display devices

ABSTRACT

A method of forming an active matrix, light emitting diode (LED) array includes removing, from a base substrate, a layer of inorganic LED material originally grown thereupon; and bonding the removed layer of inorganic LED material to an active matrix, thin film transistor (TFT) backplane array.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/303,486, filed Nov. 23, 2011, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to semiconductor devicemanufacturing and, more particularly, to methods of integrating activematrix, inorganic light emitting diodes for display devices.

Organic light emitting diode (OLED) displays have gained significantinterest recently in flat panel display applications in view of theirfaster response times, larger viewing angles, higher contrast, lighterweight, lower power, amenability to flexible substrates, as compared toliquid crystal displays (LCDs). Despite the OLED's demonstratedsuperiority over the LCD, there still remain several challenging issuesrelated to encapsulation and lifetime, yield, color efficiency, anddrive electronics, all of which are receiving considerable attention.

Nonetheless, OLEDs continue to be widely investigated as a futuretechnology choice for manufacturing flexible active matrix displays.Although passive matrix addressed OLED displays are already in themarketplace, they do not support the resolution needed in the nextgeneration displays, since high information content (HIC) formats areonly possible with the active matrix addressing scheme. Active-matrixrefers to the combination of the switching and/or driving active devices(typically thin-film transistors) and the passive devices (such as LEDs)controlled by the active devices. The active and passive parts of theactive matrix are typically referred to as the backplane and frontplane, respectively.

In contrast to the more established inorganic LEDs, the low depositiontemperature of organic materials is well compatible with low-costflexible substrates. On the other hand, the lifetime and efficiency ofcurrently existing organic LEDs is far below that of the inorganic LEDs.

SUMMARY

In an exemplary embodiment, a method of forming an active matrix, lightemitting diode (LED) array includes removing, from a base substrate, alayer of inorganic LED material originally grown thereupon; and bondingthe removed layer of inorganic LED material to an active matrix, thinfilm transistor (TFT) backplane array.

In another embodiment, a method of forming a display device includesgrowing one or more layers of inorganic light emitting diode (LED)material on a base substrate; separating the one or more layers of theinorganic LED material from the base substrate by a stress-inducedspalling technique; and bonding the separated one or more layers ofinorganic LED material to an active matrix, thin film transistor (TFT)backplane array, thereby defining an active matrix LED array.

In another embodiment, an active matrix, light emitting diode (LED)array includes a layer of inorganic LED material bonded to an activematrix, thin film transistor (TFT) backplane array.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a flow diagram illustrating a method of forming an activematrix, inorganic LED array in accordance with an exemplary embodiment;

FIG. 2(a) is a cross sectional view of a grown, inorganic LED deviceused in the method of FIG. 1;

FIG. 2(b) is a cross sectional view of an alternative embodiment of thegrown, inorganic LED device of FIG. 2(a);

FIG. 3 is a cross sectional view illustrating the formation of one ormore stress layers and a flexible handle layer on a grown, inorganic LEDdevice;

FIG. 4 is a cross sectional view illustrating mechanical force inducedspalling of the LED device along a fracture plane by lifting of thestress layer(s) via the flexible handle layer;

FIG. 5 is a scanning electron micrograph (SEM) image demonstrating asemiconductor layer spalling technique;

FIG. 6 is a schematic diagram illustrating an active matrix, TFTbackplane array to which the spalled LED layers of the LED device may bebonded;

FIG. 7 is a cross sectional view illustrating mechanical bonding ofspalled inorganic LED device to an active matrix TFT backplane array;

FIG. 8 is a cross sectional view illustrating multiple patternedinorganic LED devices bonded to an active matrix TFT backplane array forsubpixel definition;

FIG. 9 is a cross sectional view of a single inorganic LED device bondedto an active matrix TFT backplane array, using color filters forsubpixel definition; and

FIG. 10 is a cross sectional view of an alternative embodiment of asingle inorganic LED device bonded to an active matrix TFT backplanearray, using color filters for subpixel definition.

DETAILED DESCRIPTION

Disclosed herein is a method of integrating inorganic light emittingdiodes (LEDs) onto a thin-film transistor (TFT) backplane for realizingactive matrix LED displays on substrates, whether flexible or rigidsubstrates. In brief, the embodiments herein utilize a stress-induced,substrate spalling technique to transfer a layer of a conventionallygrown, inorganic LED device onto an integrated TFT backplane using amechanical bonding technique (such as cold welding, for example) tocreate an active matrix, inorganic LED array. Previously, any practicalapproach to integrating LED devices with TFT backplane substrates wasthrough the low temperature deposition of the aforementioned organic LEDmaterial.

Referring initially to FIG. 1, there is shown a flow diagramillustrating a method 100 of forming an active matrix, inorganic LEDarray in accordance with an exemplary embodiment. FIG. 1 may bereferenced again later as specific operations depicted in the flowdiagram blocks are described in further detail by way of the additionalfigures. As shown in block 102, an LED device is formed by growinginorganic LED layers on a base substrate. This initial LED deviceformation may be formed in accordance with techniques known in the art,such as by forming, for example, a gallium nitride (GaN) LED structureon a base sapphire substrate. Then, as indicated in block 108, the LEDlayers (e.g., p-type layer/active layer/n-type layer) comprising the LEDdevice are removed from the base substrate upon which they were grown bya stress-induced spalling technique. The LED layers thus transferredfrom the base substrate by spalling are then bonded to an active matrix,TFT backplane array as shown in block 108, such as by cold welding.

For a display device having large numbers of colored subpixels, it iscontemplated that the single planar LED device bonded to the backplanearray may be provided with individual color filters. As indicated indecision block 110, where color filtering is to be used, color filtersare formed as shown in block 112. On the other hand, if color filtersare not used, another option is to pattern (e.g., through conventionallithography) the LED layers of a first semiconductor type, as depictedin block 114. Thereafter, the LED growth/spalling/bonding/patterningprocessing may be repeated (i.e., the process loops back to block 102)to create an adjacent subpixel with a different light emission color.Also in the case where color filters are to be formed, the transferredLED layers could also be patterned to improve the mechanical flexibilityof the active matrix in case a flexible TFT backplane array is used.

FIGS. 2(a) and 2(b) illustrate exemplary embodiments of an LED device200 a, 200 b, respectively, in further detail. As is shown, the LEDdevice 200 a of FIG. 2(a) includes a base substrate 202 (e.g., sapphire)upon which an optional buffer layer or layers 204 (e.g., aluminumnitride) may be formed. A p-type doped semiconductor layer 206 (e.g.,GaN) is formed on the buffer layer(s) 204, an active layer or region 208(e.g., InGaN) is formed on the p-type doped semiconductor layer 206, andan n-type doped semiconductor layer 210 (e.g., GaN) is formed on theactive layer 208. The dashed line 212 represents the desired locationwhere the LED layers are spalled so as to be removed from the basesubstrate 202 and buffer layer(s) 204. In the LED device 200 b of FIG.2(b), the polarity of the LED is merely reversed, in that n-type dopedsemiconductor layer 210 is formed on the buffer layer(s) 204, followedby the active layer 208 and p-type doped semiconductor layer 206.

Referring now to FIG. 3, there is shown a cross sectional view ofillustrating the formation of one or more stress layers and a flexiblehandle layer on a grown, inorganic LED device. In this example, the LEDdevice 200 b of FIG. 2(b) is illustrated as an exemplary sourcesubstrate for the spalling process thereof. In particular, one or morethin, low-cost stressor layers (optional adhesion layer 302 and metallayer 304) are deposited on the source (LED) substrate 200 b. A flexiblehandle layer 306 is then attached to the metal stressor layer 304.

The optional adhesion layer 302 may be formed from a metal such as, forexample, titanium (Ti), tungsten (W), chromium (Cr), nickel (Ni), andalloys thereof. The metal stress layer 304 may include a metal such asNi, Cr and iron (Fe) formed opposite an interface between the adhesionlayer 302 and the source substrate 200 b. The flexible handle layer 306(such as a polyimide for example) has a suitable radius of curvaturesuch that the handle layer 306 is not too rigid so as to compromise thespalling process.

Thicknesses of the thin stressor layer or layers 302, 304, singly, or incombination are selected so as to prevent spontaneous spalling of thesource substrate 200 b simply due to the deposition of the stressorlayers themselves. Rather, it is intended that the spalling process beperformed in a controlled manner such that application of a mechanicalforce on handle layer 306 results exfoliation of a portion of the LEDsource substrate along the intended location 212, as shown in FIG. 4. Inone exemplary embodiment, the thickness of the exfoliated semiconductorlayer 304 from the source substrate 200 b is roughly twice the thicknessof the combined thicknesses of the stressor layer 304 and the optionaladhesion layer 302. By controlling the amount of strain in the stressorlayer 304, the operable thickness value of the stressor layer 304 can bechosen to remove a controlled thickness of the exfoliated LEDsemiconductor layers 308.

By way of further illustration, FIG. 5 is a scanning electron micrograph(SEM) image demonstrating a semiconductor layer spalling technique, inwhich approximately a 2 micron (μm) thick GaN layer of is released froma GaN LED structure grown on a sapphire substrate. Additional detailsregarding controlled semiconductor layer spalling using a metal stressorlayer(s) and handle layer may be found in published application U.S.2010/0311250, assigned to the assignee of the present application, andthe contents of which are incorporated herein in their entirety.

With respect to the TFT backplane array to which removed LED devicelayer may be bonded, FIG. 6 is a schematic diagram illustrating anactive matrix, TFT backplane array 600 to which the spalled LED layersof the LED device may be bonded. While it will be understood that anactual LED array will have many more pixel structures, the illustrativearray 600 is depicted as a 3×2 active-matrix LED array with a 2-TFTpixel structure. Each pixel 602 includes, for example, a switching TFT604, a driver TFT 606, a storage capacitor 608 and an LED 610, in itssimplest form. Other backplane designs, such as those used foractive-matrix OLED displays, may also be utilized as well.

Referring now to FIG. 7, there is shown a cross sectional viewillustrating mechanical bonding of spalled inorganic LED device to anactive matrix TFT backplane so as to define an active matrix LED array700. In the embodiment shown, the active matrix TFT backplane isgenerally indicated at 702, and the LED device bonded thereto isgenerally indicated at 704. Again, the specific polarity of theillustrated LED 704 is only exemplary in nature. As further depicted inFIG. 7, the array 700 also includes a contact (bottom) electrode 706used to contact the driver TFT in the backplane 702 to the LED 704. Thecontact electrode 706 may be a transparent conductive oxide (TCO),metal, or a TCO/metal bilayer (wherein a thin metal layer may bedeposited on the TCO to enhance the bonding quality if necessary). A topelectrode 708 is also formed on the LED 704, wherein the top electrodemay be formed from the same or different electrode materials withrespect to the contact electrode 706. Depending on whether a metal(opaque) or a TCO (transparent) material is used as the top and/orbottom electrode, the display may be top emitting, bottom emitting, orboth.

It will also be noted that the handle layer used for spalling (e.g.,layer 306 of FIGS. 3 and 4, not shown in FIG. 7) may or may not beremoved from the top of the active matrix after bonding.

As stated earlier, once bonded to the backplane, the LED layer may bepatterned, followed by repeating the spalling/bonding/patterningprocedure for a different LED layer to create an adjacent pixel with adifferent light-emission color such as depicted by the active matrix LEDarray 800 of FIG. 8. Here, the active matrix TFT backplane 802 has afirst patterned LED 804 a and a second patterned LED 804 b, where theLEDs 804 a, 804 b have different types of semiconductor materials thatmay be selected to provide two different color emissions. As should beappreciated, still an additional LED device(s) may be formed on thebackplane 802 in addition to LEDs 804 a, 804 b, to define a singlesubpixel, such as for an RGB or RGBW color display. Although manufactureof the array 800 is more complex in terms of the number of processingsteps, the individual LED materials provide an energy efficiencyadvantage, in that no color filtering is needed.

In lieu of patterning and repeating LED layer spalling and bonding, asingle transferred LED layer may serve as the basis of a display, inwhich the LED layer remains unpatterned on the backplane. As depicted inthe active matrix LED array 900 of FIG. 9, the active matrix TFTbackplane 902 has a single or blanket LED structure 904 bonded thereto,such as described above. Color filters 906 a and 906 b are provided overthe top side of the LED structure 904 opposite the backplane substratein accordance with a top-emission display. Here, patterning is notnecessary due to the nature of conduction of the LED, in that there ispoor lateral conduction through the LED semiconductor layers. Again, itwill be appreciated that although FIG. 9 depicts a pair of color filters906 a, 906 b, three or more such color filters could be used to form asingle pixel structure.

Finally, FIG. 10 illustrates an alternative embodiment of the array ofFIG. 9, in which for a bottom-emission display, the color filters 906 a,906 b are located at the bottom of the backplane 902. Still anotheralternative may include a double-sided emission (bifacial) display, inwhich color filters are located on both sides of the display. In anycase, additional passivating/protecting/optical layers (not shown) asknown in the art may be located between the color filters and theTCO/metal (on the top) and the substrate (on the bottom).

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

The invention claimed is:
 1. An active matrix, light emitting diode(LED) array, comprising: a first subpixel unit having layer of a firstinorganic LED material bonded to an active matrix, thin film transistor(TFT) backplane array; and a second subpixel unit having a layer of asecond inorganic LED material bonded to the active matrix, TFT backplanearray; the first inorganic LED material is different from the secondinorganic LED material, the first subpixel unit having a different lightemission characteristic than the second subpixel unit.
 2. The array ofclaim 1, further comprising a plurality of color filters formed on atleast one of a top and bottom surface of the active matrix TFT backplanearray.
 3. The array of claim 1, wherein the inorganic LED materialcomprises gallium nitride (GaN).
 4. The array of claim 1, furthercomprising a bottom contact electrode disposed between the backplanearray and the inorganic LED material.
 5. The array of claim 4, whereinthe bottom contact electrode is one or more of: a transparent conductiveoxide (TCO), a metal, and a TCO/metal bilayer.
 6. The array of claim 5,further comprising a top contact electrode formed on a top surface ofthe inorganic LED material.
 7. The array of claim 6, wherein the topcontact electrode is one or more of: a transparent conductive oxide(TCO), a metal, and a TCO/metal bilayer.
 8. The array of claim 1,wherein the layer of inorganic LED material is attached to a flexiblehandle layer, the flexible handle layer being configured to facilitate aspalling process within the inorganic LED material that exfoliates theinorganic LED material from a base substrate by application of amechanical force on the flexible handle layer.
 9. The array of claim 8,wherein the flexible handle layer comprises a polyimide material. 10.The array of claim 8, further comprising one or more stress layersformed between the inorganic LED material and the flexible handle layer.11. The array of claim 10, further comprising an adhesion layer thatforms an interface between the inorganic LED material and the one ormore stress layers.
 12. The array of claim 11, wherein the one or morestress layers comprise metal.
 13. The array of claim 12, wherein athickness of the one or more stress layers is selected so as exfoliatethe inorganic LED material from the base substrate at a desired locationwith respect to thickness of the exfoliated inorganic LED material.